Anaxogoras ca 320 BC believed the Sun was a fiery stone as were the stars and other planets. Aristotle proposed the Sun was made of a Fire essence, and the stars and planets were made from a 5th essence called Aether. From what I can tell, it wasn't until Giordano Bruno in 1583 that stars were equated as suns in their own right, with orbiting planets a la Copernicus. William Herschel in the early 1800's assumed that stars were equally bright as the Sun in order to estimate the size of the universe. It wasn't really until Bessel used parallax to determine their true distances in 1838, and in 1814-1829 Fraunhofer, Foucault, Lockyer and others used the newly invented 'stellar spectroscope' to show that the Sun and stars are made of a glowing gas, that the actual nature of stare was nailed down.